1. Field of the Invention
The present invention generally relates to a light emitting element driving circuit and a light emitting device having such a driving circuit. More particularly, the present invention is concerned with a light emitting element driving circuit capable of driving a light emitting element at a high speed without being affected by noise when the light emitting element is driven with a positive power supply voltage.
Recently, light emitting elements such as semiconductor laser diodes have been widely used in various fields such as optical communications and optical disk drives. For example, a light repeater employs a light emitting element such as a semiconductor laser diode in order to convert an electric signal to a light signal. A light emitting element driving circuit functions to drive the light emitting element. When the light emitting element is formed of a semiconductor laser diode, the light emitting element driving circuit (that is, the semiconductor laser diode driving circuit) supplies the laser diode to a pulse (driving) current corresponding to a signal to be transmitted and a bias current which defines the threshold level of the laser diode. Conventionally, a negative power supply is used to drive the light emitting element so that a current is drawn from the light emitting element to the negative power supply. Recently, there has been a trend to use a positive power supply in which a current is supplied to the light emitting element from the positive power supply. Generally, use of such a positive power supply increases the operating speed.
2. Description of the Related Art
FIG. 1 is a diagram of a conventional light emitting element driving circuit, which employs a negative power supply to which the current is drawn from the light emitting element. A light emitting element 13 has an anode connected to ground (GND), and a cathode connected to a light emitting element driving circuit 11. A constant-current source 12 has a first terminal connected to the driving circuit 11, and a second terminal connected to a negative power supply VSS. An input signal DATA is applied to the driving circuit 11, which turns ON and OFF the light emitting element 13. The driving circuit 11 draws the current from the light emitting element 13 to the negative power supply VSS. The current drawn from the light emitting element 13 includes a pulse current Ip and a bias current Ib.
FIG. 2 is a diagram of another arrangement of the light emitting element driving circuit 11 shown in FIG. 1. The cathode of the light emitting element 11 is connected to the negative power supply VSS, and the anode thereof is connected to the light emitting element driving circuit 11. The light emitting element shown in FIG. 2 includes a PNP transistor 14 and a resistor R. The emitter of the transistor 14 is coupled to the ground GND via the resistor R, and the collector thereof is connected to the anode of the light emitting element 13. The input signal DATA is applied to the base of the transistor 14. The light emitting element driving circuit 11 supplies the current to the light emitting element 13 from the ground. In this regard, the type of the driving circuit 11 shown in FIG. 2 differs from that of the driving circuit 11 shown in FIG. 1.
It should be noted that the circuits shown in FIGS. 1 and 2 are designed taking into consideration use of the negative power supply VSS. However, it may be considered that a positive power supply VCC is replaced by the ground GND as shown in parentheses shown in FIGS. 1 and 2, and the ground GND is replaced by the negative power supply VSS as shown in parentheses.
However, the above arrangement using the positive power supply VCC will have the following problems. In the circuit shown in FIG. 1, the driving circuit 11 functions to draw the current from the light emitting element 13 connected to the positive power supply VCC. In this case, neither the cathode of the light emitting element 13 nor the anode thereof is connected to the ground GND, and thus the light emitting element 13 is in a floating state. Hence, it is liable to be affected by noise such as power supply noise, so that the circuit shown in FIG. 1 does not have a good reliability of the operation thereof.
The circuit shown in FIG. 2 requires the PNP transistor 14, which operates slowly, as compared with an NPN transistor.